Increased amounts of α-methylstyrene (AMS) may be produced in a combined phenol and acetone plant by selectively reducing cumene hydroperoxide (CHP) to dimethyl phenyl carbinol (DMPC), the hydrated form of AMS. DMPC is also known as 2-phenyl-2-propanol or dimethylbenzyl alcohol (DMBA). As a byproduct of the oxidation of cumene to CHP, small amounts of DMPC are produced. The DMPC undergoes dehydration in the presence of an acid catalyst to yield AMS. Some phenol manufacturers recover the AMS if it is produced in sufficient quantities. Other phenol manufacturers do not recover AMS, but hydrogenate it back to cumene for recycle to the oxidation reactor. The hydrogenation of the AMS may take place after recovery of an AMS/cumene stream in the distillation section of the phenol plant. As an alternative approach, the entire cleavage reactor effluent, including all of the AMS may be hydrogenated prior to separation of the phenol and acetone in the distillation section of the plant, e.g., U.S. Pat. No. 5,245,090.
AMS is used industrially in a variety of applications, particularly in the production of certain copolymers and specialty polymers. In addition, AMS finds utility as an intermediate in the production of fine chemicals such as unsaturated AMS dimers. These dimers are used as molecular weight controlling agents in the production of copolymers, such as acrylonitrile-butadiene-styrene resins and styrene-butadiene rubber. The hydrogenated forms of AMS dimers are of industrial value as components in lubrication compositions.
A number of patented processes have been developed in an attempt to increase the AMS yield in the production of phenol from cumene. These processes typically seek to increase the AMS yield by minimizing the loss of AMS through secondary reactions. One approach employs a multi-step process that reacts CHP and DMPC with sulfuric acid in a back mixing reactor to produce dicumyl peroxide that subsequently undergoes decomposition at elevated temperature under plug-flow conditions to produce AMS, phenol, and acetone e.g., U.S. Pat. No. 4,358,618. An alternative approach to minimize the loss of AMS through secondary reactions employs a multi-step process that decomposes the CHP in a back mixing reactor followed by dehydration of the DMPC in a plug-flow reactor after an inhibitor such as acetone and/or water has been added to control secondary reactions of AMS, e.g., U.S. Pat. Nos. 5,998,677 and 5,463,136. These processes, however, do not increase the yield of AMS over the theoretical maximum that can be obtained by full dehydration of the DMPC produced in the oxidizer unit. These processes merely seek to minimize the loss of AMS to heavy byproducts, and result in AMS yields of 70–80% of the theoretical maximum based on the DMPC exiting the oxidizer unit.
The process of this invention provides a method for increasing the AMS yield above the theoretical maximum based on the DMPC in the oxidizer effluent by reducing a portion of the CHP stream to DMPC over a suitable heterogeneous catalyst. This process stream, having elevated amounts of DMPC, can then be fed to the cleavage unit of a phenol plant where the remaining CHP undergoes acid-catalyzed decomposition to phenol and acetone. During the decomposition, the acid catalyst dehydrates the DMPC to AMS. By controlling the fraction of the CHP reduced to DMPC, the amount of AMS produced in the plant can be continuously set to meet the demand of the market.
The process of this invention also provides a method for controlling the AMS yield by reacting a portion of the CHP with an exogenous source of propylene in an epoxidation reaction. In the epoxidation reactor, a portion of the CHP is reduced in the presence of an epoxidation catalyst to DMPC with propylene going to propylene oxide. The propylene oxide can be recovered as a valuable byproduct. The liquid solution leaving the epoxidation reactor has elevated amounts of DMPC, and can then be fed to the cleavage unit of a phenol plant where the remaining CHP undergoes acid-catalyzed decomposition to phenol and acetone. During the decomposition, the acid catalyst can dehydrate DMPC to AMS. By controlling the fraction of the CHP reduced to DMPC, the amount of AMS produced in the plant can be continuously set to meet the demand of the market for AMS.
No process currently exists that allows a phenol manufacturer to make on-demand AMS. AMS may only be recovered from dehydration of DMPC produced in the oxidation reactor. Current processes do not allow flexibility in controlling the amount of DMPC produced via oxidation as unfavorable side reactions lead to higher quantities of acetophenone when DMPC levels are increased by oxidation. Numerous methods have been disclosed in the patent literature to maximize the yield of AMS, regardless of whether AMS is recovered as a separate product or hydrogenated to cumene and recycled. These methods for improving the AMS yield seek to minimize side reactions of the AMS by using acetone as a solvent to dilute the AMS or using alternative reactor configurations. Regardless of the method used, the maximum AMS yield for current state-of-the-art plants is typically within the range of 70–80% based on the DMPC leaving the oxidation reactor.